The present invention relates to an apparatus for molding glass articles having high configuration precision or accuracy and fine surface roughness so that grinding or polishing after press-molding can be dispensed with.
In recent years, studies and researches have been conducted in which particular material is used to form a mold having an optical mirror surface, and press-molding is carried out within non-oxidizing atmosphere, thereby obtaining lenses having an optical mirror surface necessitating no grinding or polishing after the press-molding. In this connection, U.S. Pat. No. 3,833,347 discloses the use of glassy carbon as the material of the mold. It is disclosed in U.S. Pat. No. 4,139,677 to employ SiC or Si.sub.3 N.sub.4 as the material of the mold. Further, it is disclosed in U.S. Pat. No. 4,168,961 to utilize mixture of SiC and carbon as the material of the mold. According to the molding method disclosed in these U.S. patents, the mold is first heated to a temperature level approximate to a glass softening point, that is, a Litolton point at which the temperature level corresponds to the viscosity of 10.sup.7.65 poises. A load is then applied to a glass preform within the mold, to carry out molding of the glass preform. Subsequently, the mold is cooled to a glass transition temperature or below, while the load is maintained applied to the pressed glass in order to prevent the pressed glass from being deformed. After the pressed glass has been solidified completely, the load is removed from the pressed glass. Subsequently, the mold is cooled to a low temperature level of 300 degrees C, and then the mold is opened. It is required for such molding method to carry out the above various processings at the same position, resulting in such a problem that the molding cycle time is lengthened considerably, and the molding speed is low.
Further, Japanese Patent Application Laid-Open No. 59-152229 discloses an apparatus in which various processing chambers including a heating chamber, a press chamber, a cooling chamber and the like are arranged linearly. A straight rail is arranged which extends through the processing chambers. A plurality of molds are arranged on the rail. A push rod pushes the molds to move the same along the rail, thereby successively molding the glass preforms within the respective molds. The apparatus is advantageous in that the molding speed is enhanced, because the molds are moved successively through the various processing chambers to process the glass preform continuously. However, the apparatus has the following problems in relation to the temperature distribution of the molds and the movement thereof.
That is, since each mold is mounted directly on the rail, heat is transferred from the rail brought to a considerably high temperature level, to a drag or a lower mold portion of the mold. Further, since the mold is arranged at a low location within the processing chamber, which is low in temperature, the drag and a cope or an upper mold portion of the mold are different in temperature from each other. Thus, the glass preform accommodated in the mold cannot be heated or cooled uniformly. This results in such problems that moldability of the glass preform is poor and, in particular, recesses are generated in the molded glass by shrinkage thereof.
Moreover, in the apparatus disclosed in the above Japanese patent, a single push rod is arranged within an insertion chamber through which the mold is introduced into the processing chamber from the outside of the apparatus. The mold is moved onto the guide rail by the single push rod. The mold mounted on the guide rail is pushed by the following mold, and advances through the processing chamber. It is required for such transport arrangement to transport the molds while overcoming the sliding friction between the rail and the molds mounted directly on the rail. That is, large urging force overcoming the maximum static friction force is required to be applied to the molds at each time of the start-up of the intermittent transportation. Thus, movement of the molds become awkward so that shocks occur at the start-up of the transportation. If the pressed glass just after the press-molding is subjected to such shocks, distortion occurs in the glass preform or pressed glass.
Furthermore, a glass article molding apparatus of rotary type is known from Japanese Patent Application Laid-Open No. 61-26528. In the apparatus, a plurality of processing chambers such as a heating chamber, a press chamber, a cooling chamber and the like are arranged circumferentially along a circle. A plurality of molds are mounted respectively on outer ends of respective support arms which extend radially from a central rotary shaft, such that the molds are passed successively through the processing chambers. The processing chambers are defined in such a manner as to be surrounded by upper and lower walls and radially inward and outward side walls or partition walls of a furnace body. The inward side wall is formed therein with a circumferential slit. The support arms extend into the processing chambers through the circumferential slit.
Since, for the above apparatus, the circumferentially extending slit is required to be formed in the inward side wall, a heater cannot be arranged at the region of the slit. By this reason, it is impossible to uniformly heat the mold and a glass preform accommodated therein. Further, because heat escapes from the processing chamber to the outside through the circumferential slit the temperature distribution within the processing chamber is not uniform.
In general, it is necessary to sufficiently uniformize the temperature distribution of each mold and the glass preform therein, in order to mold a glass article which does not require further preparing operation such as grinding or polishing after pressing. Otherwise, thermal shrinkage of the glass is unbalanced, making it impossible to obtain a glass article having high configuration accuracy, that is, high surface accuracy. Accordingly, the nonuniformity in the heating of the mold and the glass preform therein and the nonuniformity in the temperature distribution within the processing chamber, in the above-described conventional apparatus, are extremely disadvantageous in molding of the glass article of the aforesaid type.
Moreover, in the glass article molding apparatus of rotary type mentioned above, since the support arms extend radially into the processing chambers of high temperature atmosphere, the support arms are deformed by heat, so that the molds resting on the respective support arms are not brought to a predetermined pressing position. Accordingly, pressing force cannot accurately be applied to the mold at pressing. This results in deviation or shift and inclination of the central axes of the respective cope and drag of the mold, in turn, in deviation and tilt of the optical axis of the glass article.
Furthermore, since the arrangement is such that the support arms are rotated about the axis of the central shaft to move the molds, shocks are less or small which occur due to the static friction at the start-up of transportation between the processing chambers. Since, however, the support arms are low in strength, vertical vibration may occur, resulting in distortion in the glass article.